This invention relates to an expandable orthopedic drill for facilitating placement of grafts of bone and/or intervertebral implant devices, thereby facilitating the development of a bony union between them and thus long term spinal stability.
Of all animals possessing a backbone, human beings are the only creatures who remain upright for significant periods of time. From an evolutionary standpoint, this erect posture has conferred a number of strategic benefits, not the least of which is freeing the upper limbs for purposes other than locomotion. From an anthropologic standpoint, it is also evident that this unique evolutionary adaptation is a relatively recent change, and as such has not benefitted from natural selection as much as have backbones held in a horizontal attitude. As a result, the stresses acting upon the human backbone (or "vertebral column"), are unique in many senses, and result in a variety of problems or disease states that are peculiar to the human species.
The human vertebral column is essentially a tower of bones held upright by fibrous bands called ligaments and contractile elements called muscles. There are seven bones in the neck or cervical region, twelve in the chest or thoracic region, and five in the low back or lumbar region. There are also five bones in the pelvic or sacral region which are normally fused together and form the back part of the pelvis. This column of bones is critical for protecting the delicate spinal cord and nerves, and for providing structural support for the entire body.
Between the vertebral bones themselves exist soft tissue structures, discs, composed of fibrous tissue and cartilage, which are compressible and act as shock absorbers for sudden downward forces on the upright column. The discs allow the bones to move independently of each other, as well. The repetitive forces which act on these intervertebral discs during day-to-day activities of bending, lifting and twisting cause them to break down or degenerate over time.
Presumably because of humans' upright posture, their intervertebral discs have a high propensity to degenerate. Overt trauma, or covert trauma occurring in the course of repetitive activities, disproportionately affect the more highly mobile areas of the spine. Disruption of a disc's internal architecture leads to bulging, herniation or protrusion of pieces of the disc and eventual disc space collapse. Resulting mechanical and even chemical irritation of surrounding neural elements (spinal cord and nerves) cause pain, attended by varying degrees of disability. In addition, loss of disc space height relaxes tension on the longitudinal spinal ligaments, thereby contributing to varying degrees of spinal instability such as spinal curvature.
The time-honored method of addressing the issues of neural irritation and instability resulting from severe disc damage have largely focused on removal of the damaged disc and fusing the adjacent vertebral elements together. Removal of the disc relieves the mechanical and chemical irritation of neural elements, while osseous union (bone knitting) solves the problem of instability.
While cancellous bone appears ideal to provide the biologic components necessary for osseous union to occur, it does not initially have the strength to resist the tremendous forces that may occur in the intervertebral disc space, nor does it have the capacity to adequately stabilize the spine until long term bony union occurs. For these reasons, may spinal surgeons have found that interbody fusion using bone alone has an unacceptably high rate of bone graft migration or even expulsion or nonunion due to structural failure of the bone or residual degrees of motion that retard or prohibit bony union. Intervertebral prostheses in various forms have therefore been used to provide immediate stability and to protect and preserve an environment that fosters growth of grafted bone such that a structurally significant bony fusion can occur.
A limitation of present interbody fusion techniques is that the vertebral bodies are distracted prior to the drilling of the end plates, necessitating a large diameter drill in situations where ligamentous laxity permits a wide distraction of the intervertebral space. A large diameter drill in turn requires greater retraction of neural or vascular elements, thereby increasing traction-related complication rates. Large diameter drills are also counter-productive for situations requiring endoscopic or "minimal access" techniques.
The present invention allows insertion of the drill into an undistracted disc space such that the drilling to prepare the end plates can be done by an instrument that expands wholly within the protective confines of the disc space without requiring significant distraction of vertebral body elements. By doing this, neural and vascular elements are protected from injury by the cutting surface of the drill and minimal retraction of these elements is required because distraction of the space is not required. In addition, only a single instrument is required to adapt for any size disc space, in contrast to present techniques which require a variety of drills that increase in diameter by 2 mm increments. By varying the lengths of links supporting the cutters, the cutting angle of the drill can be altered to facilitate the production of fluted or angled disc spaces in extremely kyphotic or lordotic spines. In addition, the cutting edge of the drill is serrated to maximize the surface area of the cut bone to promote bony union.